Rhodium-catalyzed intramolecular cyclization of naphthol- or phenol-linked 1,6-enynes through the cleavage and formation of sp2 C-O bonds

Article abstract of DOI:10.1002/anie.201201186

Put a ring on it: A cationic rhodium(I)/binap complex catalyzes the intramolecular cyclization reactions of naphthol- or phenol-linked 1,6-enynes to produce vinylnaphtho- or vinylbenzofurans and vinylnaphtho- or vinylbenzopyrans through the cleavage and f

Full text of DOI:10.1002/anie.201201186

Angewandte
Chemie
DOI: 10.1002/anie.201201186
Organometallic Catalysis
Rhodium-Catalyzed Intramolecular Cyclization of Naphthol- or
Phenol-Linked 1,6-Enynes Through the Cleavage and Formation of sp2
À
C O Bonds**
Norifumi Sakiyama, Keiichi Noguchi, and Ken Tanaka*
Recently, a number of p-electrophilic transition-metal-cata-
lyzed intramolecular migratory cycloisomerization reactions,
which involve sp3 carbon–heteroatom bond cleavage, have
been developed for the synthesis of five-membered hetero-
cycles.[1] In these reactions, copper, silver, gold, palladium,
and platinum complexes were frequently employed because
of their high catalytic activity for p-bond activation.[2–6] For
example, the transition-metal-catalyzed intramolecular cycli-
zation of 2-alkynylphenyl ethers leading to benzofuran
derivatives was reported (Scheme 1).[2] In this reaction, the
p-electrophilic transition-metal catalyst activates the alkyne
p-bond, which generates an (alkenyl)metal intermediate
bearing an allyl or benzyl cation fragment.[2] This cation
fragment shifts to the most nucleophilic b-position relative to
the oxygen atom, which affords the benzofuran derivative and
regenerates the transition-metal catalyst.[2] Accordingly, this
mode of cyclization is limited to the cleavage and formation
enynes possessing a vinyl ether moiety would react with the
rhodium(I) complex to generate the corresponding rhoda-
cycles, which would undergo b-hydride or oxygen elimination
(Scheme 2).[10–12]
We first investigated the reaction of 1-hexynyl-2-naphthyl
vinyl ether (1a) in the presence of a cationic rhodium(I)/
binap complex (10 mol%) at 808C. We were pleased to find
of sp3 C O bonds by the generation of stabilized cation
À
fragments (Scheme 1).[2] Herein, we disclose the unprece-
dented transition-metal-catalyzed intramolecular cyclization
of naphthol- or phenol-linked 1,6-enynes leading to vinyl-
naphthofuran or vinylbenzofurans through the cleavage and
2
[7,8]
À
formation of sp C O bonds.
We recently reported an olefin isomerization/enantiose-
lective intramolecular Alder-ene reaction cascade of phenol-
or naphthol-linked 1,7-enynes catalyzed by a cationic rho-
dium(I)/(R)-binap complex (Scheme 2).[9] This cascade reac-
tion proceeds through the formation of phenol- or naphthol-
linked 1,6-enynes, which possess an isobutenyl ether moiety,
followed by rhodacycle formation and b-hydride elimina-
tion.[9] We anticipated that phenol- or naphthol-linked 1,6-
Scheme 1. Transition-metal-catalyzed intramolecular cyclization of
2-alkynylphenyl ethers leading to benzofuran derivatives.
[*] N. Sakiyama, Prof. Dr. K. Tanaka
Department of Applied Chemistry, Graduate School of Engineering,
Tokyo University of Agriculture and Technology
Koganei, Tokyo 184-8588 (Japan)
E-mail: tanaka-k@cc.tuat.ac.jp
Homepage: http://www.tuat.ac.jp/~tanaka-k/
Prof. Dr. K. Noguchi
Instrumentation Analysis Center, Tokyo University of Agriculture
and Technology
Koganei, Tokyo 184-8588 (Japan)
[**] This work was supported partly by Grants-in-Aid for Scientific
Research (Nos. 23105512 and 20675002) from MEXT (Japan). We
thank Takasago International Corporation for the gift of H8-binap,
segphos, and xyl-binap, and Umicore for generous support in
supplying a rhodium complex.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201186.
Scheme 2. Rhodium-catalyzed reactions of phenol- or naphthol-linked
1,7- and 1,6-enynes. Cod=1,5-cyclooctadiene.
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that intramolecular trans carboalkoxylation product 2a was
obtained as the major product along with the minor cyclo-
isomerization product, 3a (Table 1, entry 1). Screening of
biaryl bisphosphine ligands (Figure 1; Table 1, entries 1–6)
revealed that the selectivities between 2a and 3a depend on
the dihedral angles and steric bulk of the biaryl bisphosphine
ligands (dihedral angle: H8-binap > binap > segphos >
biphep,[13] 2a/3a: biphep > segphos > binap > H8-binap;
steric bulk: xyl-binap > tol-binap > binap, 2a/3a: xyl-
binap > tol-binap > binap). Non-biaryl bisphosphine ligands
were also examined and moderate selectivities were observed
(Figure 1; Table 1, entries 7 and 8). Although biphep showed
the highest selectivity for 2a over 3a, binap showed the
highest catalytic activity. Thus, reaction conditions were
further optimized using binap as a ligand. Gratifyingly, the
use of chlorobenzene as a solvent significantly improved the
yield of 2a (entry 9). The catalyst loading could be reduced to
5 mol% (entry 10), and increasing the reaction temperature
to 908C further improved the yield of 2a (entry 11).
Importantly, platinum, gold, and palladium complexes,
which are known to catalyze the cyclization of allyl or
benzyl 2-alkynylphenyl ethers, failed to catalyze the forma-
tion of 2a and 3a (entries 12–14).[14]
With the optimized reaction conditions in hand, we
explored the scope of the cyclization of 1,6-enynes catalyzed
by a cationic rhodium(I)/binap complex, as shown in Table 2.
With respect to the substituent at the alkyne terminus,[15] not
only n-butyl (1a, entry 1), but also phenylpropyl and chlor-
opropyl substituted alkynes (1b,c, entries 2 and 3) furnished
the desired naphthofurans 2a–c in high yields. Terminal
alkyne 1d (entry 4) could be employed, although the product
yield was moderate. Alkenyl- and arylacetylene derivatives
(1e–g, entries 5–7) were also suitable substrates for
this process and gave the desired naphthofurans 2e–
g in good yields. The 1,6-enynes 1h–k, which possess
secondary alkyl groups at the alkyne termini,
afforded naphthopyranes 4h–k as major products
and naphthofurans 2h–k as minor products
Table 1: Optimization of reaction conditions for the transition-metal-catalyzed
cyclization of 1,6-enyne 1a.[a]
(entries 8–11).
However,
cyclopropylacetylene
derivative 1l (entry 12) afforded naphthofuran 2l
as the sole product. The reaction of racemic 1,6-
enyne 1i using the cationic rhodium(I)/(R)-binap
catalyst furnished enantioenriched naphthofuran 2i
and racemic naphthopyran 4i (entry 9). With respect
to the aryl ether moiety, the reactions of phenol-
linked 1,6-enynes 1m,n afforded benzofurans
2m,n[16] in low yields (entries 13 and 14). Also,
phenol-linked 1,6-enyne 1o afforded benzopyrane
4o (entry 15), although the yield was lower than for
naphthopyrane 4h (entry 8).
Entry Catalyst
Solvent
Conditions Conv. Yield[%][b]
[%]
(2a/3a)[c]
1
[Rh(cod)2]BF4/binap
(CH2Cl)2 808C, 1 h
100
80
95
95
90
52
66
85
100
100
100
100
100
0
76 (4:1)
44 (2:1)
49 (20:1)
35 (25:1)
58 (5:1)
18 (8:1)
22 (5:1)
41 (5:1)
88 (25:1)
86 (25:1)
88 (>50:1)
0
2
[Rh(cod)2]BF4/H8-binap (CH2Cl)2 808C, 1 h
3
4
[Rh(cod)2]BF4/segphos
[Rh(cod)2]BF4/biphep
(CH2Cl)2 808C, 2 h
(CH2Cl)2 808C, 1 h
5
[Rh(cod)2]BF4/tol-binap (CH2Cl)2 808C, 3 h
6
7
8
9
[Rh(cod)2]BF4/xyl-binap
[Rh(cod)2]BF4/dppb
[Rh(cod)2]BF4/dppf
[Rh(cod)2]BF4/binap
[Rh(cod)2]BF4/binap
[Rh(cod)2]BF4/binap
PtCl2
(CH2Cl)2 808C, 3 h
(CH2Cl)2 808C, 3 h
(CH2Cl)2 808C, 3 h
ClC6H5
ClC6H5
ClC6H5
(CH2Cl)2 808C, 16 h
(CH2Cl)2 808C, 16 h
THF
The double intramolecular cyclization of 1p also
proceeded to give the corresponding blue-emitting
808C, 1 h
808C, 1 h
908C, 1 h
10[d]
11[d]
12[e]
13[e]
14[e]
benzene-linked
bis(vinylnaphthofuran)
2p
(Scheme 3).[17] This product showed a high quantum
yield of 73% in CHCl3 solution and could be
employed as a light-emitting polymerization unit.
The transformation of cyclization products was
also briefly examined. Diels–Alder reactions
between naphthofurans 2a,d and N-methylmale-
imide (5) gave pentacyclic compounds 6a,d in good
yields and as single diastereomers (Scheme 4).[18] The
Diels–Alder reaction between 5 and naphthopyran
4h also proceeded to give pentacyclic compound 7 in
AuCl(PPh3)/AgBF4
Pd(PPh3)4, K2CO3
0
0
[f]
908C, 1 h
[a] Catalyst (0.010 mmol, 10 mol%), 1a (0.10 mmol), and solvent (1.0 mL) were
used. [b] Yield of isolated product. [c] Determined by H NMR spectroscopy.
[d] Catalyst (0.010 mmol, 5 mol%), 1a (0.20 mmol), and solvent (3.0 mL) were
used. [e] 20 mol% of catalyst was used. [f] 0.50 mmol. Cod=1,5-cyclooctadiene.
1
good yield and as a single diastereomer (Scheme 4).[19] The
structure of 7 was unambiguously confirmed by X-ray
crystallographic analysis.[20]
A possible mechanism for the rhodium-catalyzed cycliza-
tion reactions of 1,6-enynes 1 leading to benzofurans 2 and 3,
and benzopyrans 4 is shown in Scheme 5. The 1,6-enyne
1 reacts with rhodium to generate rhodacyclopentene A.[21]
Subsequent b-hydride elimination of the propargylic hydro-
gen affords rhodium hydride B. Reductive elimination
followed by double bond isomerization[22] affords naphtho-
furan 3. Alternatively, a b-oxygen elimination from rhodacy-
clopentene A, followed by aryl–alkenyl single bond rotation
Figure 1. Structures of bisphosphine ligands.
2
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Table 2: Cyclization of 1,6-enynes 1a–o catalyzed by a cationic RhI/binap
complex.[a]
Scheme 3. Double cyclization of 1p.
Scheme 4. Diels–Alder reactions.
[a] Reactions were conducted using [Rh(cod)2]BF4/rac-binap
(0.010 mmol, 5 mol%), and 1 (0.20 mmol) in ClC6H5 (3.0 mL) at 908C
for 1–24 h. [b] Yield of isolated product. [c] Reaction conducted at 808C.
[d] ClC6H5 (8.0 mL) was used. [e] [Rh(cod)2]BF4/rac-binap (0.020 mmol,
10 mol%) was used. [f] Catalyst: [Rh(cod)2]BF4/(R)-binap (0.020 mmol,
10 mol%). Cod=1,5-cyclooctadiene.
in zwitterionic intermediate C and cyclization of rhodium
carbene D affords oxarhodacycle E, whereupon reductive
elimination of rhodium affords benzofuran 2. When the 1,6-
enyne possesses an alkyl group at the alkyne terminus,
b-hydride elimination from intermediate E would instead
afford intermediate F. We believe that, as the formation of
oxarhodacycle E and rhodium hydride F is reversible, 1,6-
enynes 1h–k, which possess secondary alkyl groups at the
alkyne termini, would favor the formation of thermodynami-
cally stable tetrasubstituted allene intermediate F. Elimina-
Scheme 5. Possible reaction mechanism.
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tion of rhodium leads to allenyl phenol G, followed by 6-endo
cyclization to afford benzopyran 4. Alternatively, 5-exo
cyclization of intermediate G would afford benzofuran 2.
The higher yields of the cyclization products from naphthol-
linked 1,6-enynes than those from phenol-linked ones might
be attributed to the higher reactivity of an oxa-ortho-
quinodimethane version of intermediate D, generated from
the phenol-linked 1,6-enyne, versus a 1-methylenenaphtha-
len-2(1H)-one version of intermediate D, generated from the
naphthol-linked 1,6-enyne. Indeed, the reactions of 1m–o
afforded complex mixtures of products other than the desired
products (Table 2, entries 13–15).[23]
The following mechanistic studies were performed to
discard mechanistic alternatives. The reaction of benzyl
1-hexynyl-2-naphthyl ether (1q) failed to furnish naphtho-
furan 2q, which excludes a p-bond activation mechanism with
the cationic rhodium(I) complex (Scheme 6).[24]
like intermediate H, which is stabilized by the vinyl substitu-
ent, resulting in loss of configuration.[25] Thus, the axial
chirality of G would not be transferred into the central
chirality of 4i.
In conclusion, we have established that a cationic rho-
dium(I)/binap complex catalyzes the intramolecular cycliza-
tion reactions of naphthol- or phenol-linked 1,6-enynes to
produce vinylnaphtho- or vinylbenzofurans and vinylnaph-
tho- or vinylbenzopyrans through cleavage and formation of
2
À
sp C O bonds. Mechanistic studies imply that the present
cyclization reactions proceed by b-oxygen elimination from
the rhodacyclopentene intermediate. Future work will focus
on further development of novel rhodium catalyzed reactions
involving b-heteroatom elimination.
Received: February 13, 2012
Revised: March 22, 2012
Published online: && &&, &&&&
Keywords: alkynes · enynes · naphthofurans · naphthopyrans ·
.
rhodium · vinyl ethers
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(CD3OD) (Scheme 7). This result excludes the formation of
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À
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À
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3
À
As shown in entry 9 of Table 2, the reaction of racemic 1i
using the cationic rhodium(I)/(R)-binap catalyst furnished
enentioenriched naphthofuran 2i, while naphthopyran 4i was
obtained as a racemate. Therefore, a kinetic resolution occurs
in the reductive elimination and b-hydride elimination steps
from oxarhodacycle E, which affords enantioenriched 2i and
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À
amide C N bonds has been reported; see: a) T. Shimada, I.
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À
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4
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[20] CCDC 865637 (7) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
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[21] Shibata and co-workers reported the cationic rhodium(I)/tol-
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(20 mol%) proceeded at 408C to give the corresponding
[2+2+2] cycloaddition product in 27% yield.
[22] We previously reported the rhodium-catalyzed cycloisomeriza-
tion of 1,6- and 1,7-diynes leading to trienes. In this cyclo-
isomerization, isomerized triene products were obtained instead
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Lett. 2007, 9, 3953. Furthermore, we have recently demonstrated
that the isomerization of 3-alkylidene-1,2-dihydrobenzofuran to
benzofuran is rapid at 808C; see Ref. [9].
[23] The reaction of 2,3-naphthol derivative 1s was also examined,
but the desired naphthofuran 2s was obtained in very low yield
along with a complex mixture of by-products. This result might
also be attributed to the high reactivity of oxa-ortho-quinodi-
methane type intermediate D’.
[12] For cationic Pd complex-catalyzed reactions of vinyl ethers
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from this benzofuran product, a complex mixture of by-products
was generated.
[15] The reaction of enyne 1r with a cationic rhodium(I) catalyst led
to a complex mixture of products. This may be because the
expected intermediate, rhodacycle A’, possesses two possible
b-oxygen or hydride elimination sites.
[24] For examples of the p-electrophilic transition-metal-catalyzed
intramolecular cyclization of 2-alkynylphenyl benzyl ethers
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[25] Toste and co-workers reported the gold-catalyzed dynamic
kinetic asymmetric cyclization of propargyl esters to produce
benzopyrans. They proposed that the oxygen substituent of an
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cation-like intermediate, resulting in the loss of configuration;
see: Y.-M. Wang, C. N. Kuzniewski, V. Rauniyar, C. Hoong, F. D.
Toste, J. Am. Chem. Soc. 2011, 133, 12972.
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Organometallic Catalysis
N. Sakiyama, K. Noguchi,
K. Tanaka*
&&&& — &&&&
Rhodium-Catalyzed Intramolecular
Cyclization of Naphthol- or Phenol-Linked
1,6-Enynes Through the Cleavage and
Put a ring on it: A cationic rhodium(I)/
binap complex catalyzes the intramolec-
ular cyclization reactions of naphthol- or
phenol-linked 1,6-enynes to produce
vinylnaphtho- or vinylbenzofurans and
vinylnaphtho- or vinylbenzopyrans
through the cleavage and formation of sp2
2
À
À
Formation of sp C O Bonds
C O bonds. Mechanistic studies imply
that the present cyclization reactions
proceed through b-oxygen elimination
from cationic rhodacycle intermediates.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!